1. Field of the Invention
The present invention relates to optical transmission systems implementing encrypting by deterministic chaos.
2. Background Information
The very fast progress of telecommunications systems using the public optical fiber network is being accompanied by said network carrying traffic constituted by information of a private or commercial kind. It is therefore essential to have encoding means for protecting such information when it is for selective distribution. In general, signals are encrypted using highly specific codes generated by mathematical algorithms. Such methods are reserved for signals that possess a high degree of confidentiality, but they suffer from the drawback, due to their complexity with respect to encoding and decoding, of being suitable for use only at relatively low transmission rates (a few tens of Kbit/s approximately). Some applications, such as remote distribution of paid-for information or TV channels, for example, can accept a much lower degree of security, providing it is nevertheless capable of providing protection against intruders possessing ordinary detection means. Under such circumstances, the requirements are for an encrypting device that is cheap, reliable, simply reconfigurable by quickly changing the encrypting key, and capable of accommodating high data rates.
The proposed apparatus sets out to satisfy these requirements.
Proposals have already been made to use deterministic chaos phenomena for encrypting radiofrequency communications.
In this respect, reference may advantageously be made to the following publications:
[1] L. Pecora, "Overview of chaos and communications research", SPIE, Vol. 2038, p. 2, 1993; PA1 [2] S. Hayes, C. Grebogi, and E. Ott, "Communicating with chaos", Phys. Rev. Lett., Vol. 70, p. 3031, 1993. PA1 [3] P. Celka, "Chaotic synchronization and modulation of non-linear time-delayed feedback optical systems", IEEE Transactions on Circuits and Systems, Vol. 42, No. 8, p. 1, 1995; PA1 [4] Takizawa, Liu, and Ohtsubo, "Chaos in a feedback Fabry-Perot interferometer", IEEE Journal of Quantum Electronics, Vol. 30, No. 2, p. 334, 1994. PA1 transmitter means for emitting an encrypted optical signal, said means comprising means for generating deterministic chaos and means for encoding the information-carrying signal by said deterministic chaos; PA1 a line for conveying the signal encrypted in this way; and PA1 receiver means comprising means for generating deterministic chaos, and means for synchronizing said chaos with the received encrypted signal and for decrypting said encrypted signal; PA1 characterized in that the chaos is wavelength chaos, the means for generating chaos including a wavelength tunable light source and a feedback loop for feeding the light signal output by said source back to the wavelength tuning control, said loop including means for non-linear wavelength conversion, and a delay line.
The chaos generator systems used for this purpose are electronic systems in which non-linearities are produced by diodes or by comparators having thresholds and hysteresis.
Chaos phenomena have also been studied on delayed differential optical systems having non-linear dynamics, using the light power of the signal as the dynamic optical variable.
In this respect, reference may be made to the following publications: